The central processing units (CPUs) of most modern day computers are typically provided on large circuit boards (mother boards) populated with various integrated circuit (IC) components, such as microprocessors and memory devices. These components contain integrated circuits formed on semiconductor dies, generally for performing a specific function. The components work in conjunction with one another to perform the various functions of the computer. Contacts on the mother board are connected to contacts on the components by the use of multi-chip modules or directly by conventional means, such as solder. The components are connected to one another by metal patterns formed on the surface of the module, mother board or other support. These metal patterns provide a conduit for data exchange between the components.
There is a constant need for computers that operate at faster rates. In order to accommodate this need, various techniques have developed to increase the rate (bandwidth) at which data can be processed and transmitted. One of these involves increasing the circuit complexity of the integrated circuits which also often results in a larger package for the semiconductor die, and an increase in the number of input/output (I/O) terminals required for the semiconductor die. Since the amount of data that can be accessed from or transferred to a component is directly proportional to the number of I/O lines its semiconductor die contains, increasing the number of I/O terminals directly increases data transfer and processing speed.
Traditionally, semiconductor dies were connected to leads with fine wires (wire bonding). This method of connection was limited by the number of pads which could be placed on the periphery of the semiconductor die. Considerable progress has been made in reducing the semiconductor die pad size, thereby increasing the number of pads. However, this technology is still limited by the number of pads which can be formed on the die periphery, and therefore the number of I/Os on a die is likewise limited. Therefore, other techniques have been developed over the years to increase the number of available I/O terminals and while accommodating alignment problems.
One of these techniques, known as Controlled Collapse Chip Connection (C4), was developed in the 1960s to deal with the problems associated with alignment of semiconductor dies on a substrate. This process also sought to increase the number of I/O terminals which could be made available for each semiconductor die. The C4 process uses solder bumps deposited on flat contacts on the semiconductor dies to form the bond between the semiconductor die and the leads. The contacts and solder balls on the semiconductor dies are matched with similar flat contacts on the leads to form the connection. Once the die is placed on top of the contacts, the entire device is heated to a temperature which melts the solder. As the solder is allowed to set, a reliable bond is formed between the chip and the leads.
One of the main advantages of this process is that the semiconductor die self-aligns itself on the module substrate based on the high surface tension of the solder. In other words, the chip need not be perfectly aligned over the contacts of the substrate. As long as it is in close proximity, the melting of the solder will align the chip with the substrate contacts. The other advantage of this process is that an increased number of I/O terminals can be fabricated for each semiconductor die as bonding pads are not limited to the periphery of the die. This type of bonding process is also often referred to as “flip-chip” or “micro-bump” bonding. The process can be briefly explained with reference to FIGS. 1 and 2.
FIG. 1 shows a side view of a semiconductor die 10 and a support 20. The semiconductor die 10 is fabricated with various metal pattern lines and contacts 50 imprinted on its last metal level, as shown in FIG. 2. Formed beneath the semiconductor die 10 is an array of solder balls 30. The support 20 includes metallized paths 60 for carrying signals from the semiconductor die 10 to other elements mounted on the support 20. These paths have contacts which match the contacts located on the underside of the semiconductor die 10. When the semiconductor die 10 is ready to be mounted, it is placed on top of the support 20 above the support contacts. The solder balls 30 attached to the contacts of the semiconductor die rest on the contacts of the support, as shown in FIG. 1. When the device is heated, the solder melts and the semiconductor die 10 self-aligns with the support contacts. The solder later hardens to form a reliable bond between the two sets of contacts. FIG. 3 shows the device after the solder has been heated and set.
Traditionally, the contacts and solder balls have been formed on the semiconductor die using metal mask technology. In this process, a metal mask (essentially a metal plate with a pattern of holes therein) is placed over a substrate containing many semiconductor dies for forming the contacts and solder balls. Then, contact material and solder are evaporated through the holes onto the wafer. The holes in the metal masks must be of sufficient size to prevent warpage and damage of the mask during use. Hence, the number of contacts that can be fabricated through use of a metal mask is limited because the holes in the mask must remain above a minimum size to prevent these problems. Consequently, the size of the solder balls that can be created is similarly limited.
The minimum diameter of a C4 solder ball commonly achieved using current techniques, such as metal mask, is approximately 100 microns. Since the size of the solder balls is directly related to the number and density of I/O terminals that can be fabricated on a given semiconductor die, a decrease in solder ball size would provide for an increase in the number and density of the I/O terminals. This would, in turn, allow for a significant increase in data transmission rates because of the increased number of I/O ports for the packaged IC component.
Hence, there is currently a need for a process for forming solder ball contacts which are less than 100 microns in diameter.